Magnetic field sensor

ABSTRACT

The invention provides a Hall effect magnetic field sensor ( 10, 50 ) including carrier excluding or extracting means ( 36, 66 ) for reducing an intrinsic contribution to carrier concentration in the active region ( 14   e,    53   c ) to provide for the sensor to be operative in an extrinsic saturated regime. This provides an advantage that magnetic field measurement sensitivity of the sensor ( 10, 50 ) can be made substantially insensitive to sensor temperature thereby improving measurement accuracy.

[0001] This invention relates to a magnetic field sensor ofsemiconductor material.

[0002] Before considering the prior art, semiconductor properties willbe discussed. Semiconductor magnetic field sensors operate usingelectrical transport effects, and broadly speaking, there are threeimportant conduction regimes: unsaturated extrinsic, saturated extrinsicand intrinsic, and these occur at low, moderate and high temperaturerespectively. In the unsaturated extrinsic regime, there is insufficientthermal energy to ionise all impurities and the carrier concentration istemperature dependent because increasing the temperature ionises moreimpurities. Carriers are activated from dopant impurities of a singlespecies, ie donors or acceptors. Conduction is due substantially to onekind of carrier in one band, ie electrons in the conduction band orholes in the valence band but not both. The saturated extrinsic regimeis similar, but occurs at higher temperatures at which virtually allimpurities have become ionised but insufficient thermal energy isavailable to ionise significant numbers of valence band states to createelectron-hole pairs: here the carrier concentration is largelyindependent of temperature.

[0003] In the intrinsic regime, conduction has a substantialcontribution from thermal ionisation of valence band states producingboth types of carrier, ie electron-hole pairs, in addition to carriersof one type activated from impurities. Conduction is due to both kindsof carrier in both bands, ic electrons in the conduction band and holesin the valence band. Conductivity varies with temperature in this regimebecause the electron-hole pair concentration is temperature dependent.There is an intervening transition region between the extrinsic andintrinsic regimes where conduction is partially extrinsic and partiallyintrinsic giving rise to more of one type of charge carrier than theother, ie majority carriers and minority carriers: it is at or nearambient temperature in Ge depending on doping. The onset temperature ofintrinsic conduction depends on band gap and dopant concentration; itcan occur below ambient temperature, as low as 150K in narrow gapsemiconductors with low doping.

[0004] Materials such as Si and GaAs with a saturated extrinsic regimeat room temperature are preferred for magnetic field sensor applicationsdespite their inferior mobility properties: this is because of the needfor Hall effect or resistance to be largely independent of temperature.By analogy with Ge which if sufficiently purified is intrinsic atambient temperature, weakly doped Si is sometimes referred to wrongly asintrinsic, such as in PIN diodes where the high resistivity I(“intrinsic”) region is in fact extrinsic at ambient temperature. Thepurest Si currently available is more than an order of magnitude tooimpure to be intrinsic at ambient temperature.

[0005] Magnetic field sensors involving semiconductor materials havebeen known for many years. They include:

[0006] (a) magneto-resistance sensors which change in electricalresistance in response to applied magnetic field, and

[0007] (b) Hall effect sensors which respond to a magnetic field bydeveloping a voltage proportional to sensor current and field strength.

[0008] The electrical resistance R_(M) of an extrinsicmagneto-resistance sensor in a magnetic field B is given by:

R _(M) =R ₀(1+μ² B ²)  (1)

[0009] where μ is charge carrier mobility and R₀ is sensor resistance inthe absence of a magnetic field. The magneto-resistance contribution toEquation (1) is μ²B²R₀ which varies as the square of both mobility andmagnetic field.

[0010] A conventional Hall effect sensor arrangement consists of arectangular block of semiconductor material carrying a longitudinalcurrent in a transverse magnetic field: this produces a Hall voltageV_(H) orthogonal both to field and current: for an extrinsicsemiconductor arranged in this way, V_(H) is given by: $\begin{matrix}{V_{H} = {{E_{y}t_{y}} = {\frac{1}{ne}j_{x}B_{z}t_{y}}}} & (2)\end{matrix}$

[0011] where

[0012] E_(y)=Hall effect electric field;

[0013] t_(y)=semiconductor thickness dimension across which V_(H) ismeasured;

[0014] n=charge carrier concentration;

[0015] e=charge on each charge carrier (negative for electrons, positivefor holes);

[0016] j_(x)=current density in the semiconductor per unitcross-sectional area;

[0017] B_(z)=magnetic field; and

[0018] indexes x, y, z indicate x y and z co-ordinate axes anddirections of parameters to which they are suffixed.

[0019] For an extrinsic regime with one type of charge carrier, the Hallcoefficient R_(H) is defined as $\begin{matrix}{R_{H} = {\frac{E_{H}}{j_{x}B_{z}} = \frac{1}{ne}}} & (3)\end{matrix}$

[0020] The situation is more complicated than Equation (3) indicates ifthe semiconductor is in an intrinsic regime with two types of chargecarrier.

[0021] The conductivity σ of a material is given by

σ=neμ _(c)  (4)

[0022] where μ_(c) is the conductivity mobility.

[0023] A carrier mobility value μ_(H) referred to as the Hall mobilityis obtainable by multiplying Equations (3) and (4) together, ie:

μ_(H) =σR _(H)  (5)

[0024] If conduction is extrinsic, the Hall mobility differs from theconductivity mobility by a numerical factor whose magnitude depends onthe carrier scattering mechanism. However, Hall and conductivitymobilities follow the same general principles and will hereinafter betreated as equal and referred to as μ. If conduction is intrinsic theexpression for the Hall coefficient is more complex and is magneticfield dependent.

[0025] A large Hall voltage is desirable for ease of measurement; it canbe achieved by using a high current density, which requires lowresistivity to limit power dissipation and hence high carrier mobility.It is also desirable for magneto-resistance sensors to have high carriermobility to reduce resistance and hence power consumption and toincrease the sensitivity of magneto-resistance to magnetic field, whichas has been said varies as the square of mobility from Equation (1).Narrow band gap semiconductors such as InSb or InAs best satisfy thismobility criterion. InSb has an electron mobility μ_(e) of 8 m²V⁻¹s⁻,nearly ten times that of GaAs, which is 0.85 m²V⁻¹s⁻¹ and is in turnbetter than that of Si.

[0026] Despite their splendid mobility properties, narrow band gapsemiconductors are not generally used for Hall effect ormagneto-resistance sensors because they are intrinsic at ambienttemperature. This results in low Hall coefficient and Hall voltage, andin Hall voltage and sensor resistance varying with temperature; itconflicts with an important requirement of a magnetic field sensor,namely that its response to magnetic field should be relativelyinsensitive to temperature change. Another consequence of the intrinsicregime is that Hall effect is non-linear with magnetic field(magneto-resistance varies as the square of magnetic field irrespectiveof regime). These problems have placed restrictions on use of narrowband gap semiconductors in magnetic field sensors operating at roomtemperature (290K) or above: in particular, they need to be heavilydoped to reduce the temperature dependence of the carrier concentration(i.e. to make them extrinsic). This tends to defeat the object of usingthem, because it reduces their carrier mobility considerablycounteracting their advantage.

[0027] Conventional magnetic field sensors are operated in the saturatedextrinsic regime, where the carrier concentration is largely constantand does not produce unwanted changes in resistance and Hall effect.Temperature dependence of resistance and Hall effect arises however frommobility reduction with rise in temperature due to increased phononscattering and onset of electron-hole scattering.

[0028] Prior art magnetic sensors based on silicon technology tend to bephysically robust and are widely used in the motor industry in harshenvironments: They are used for example in brushless compact disc (CD)drive motors where low noise is paramount. However, they suffer from thegeneral problem of temperature dependent sensitivity, and moreover theirsensitivity is inadequate for some applications.

[0029] It is an object of this invention to provide an alternative formof magnetic field sensor.

[0030] The present invention provides a magnetic field sensorincorporating a semiconducting sensor element with an active region inwhich a signal responsive to magnetic field is developed in operation,characterised in that the sensor element:

[0031] (a) is in an at least partially intrinsic conduction regime whenunbiased and at a normal operating temperature;

[0032] (b) includes a junction which is biasable to reduce intrinsicconduction in the active region and confine charge carrierspredominantly to one type only corresponding to an extrinsic saturatedregime regime, and

[0033] (c) includes means for detecting a signal developed in the activeregion (14 e, 53 e) in response to applied magnetic field.

[0034] Biasable junctions of the kind indicated above are known per sein photodiodes from U.S. Pat. No. 5,016,073.

[0035] The invention provides the advantage that it enables magneticfield sensors to be made from a high mobility material hithertoconsidered unsuitable due to intrinsic conduction. Furthermore, in thecase of a Hall effect sensor, it is possible to obtain improvedlinearity of the Hall effect as a function of magnetic field: this is aconsequence of reducing intrinsic conduction, ie removing electrons andholes in equal numbers and changing conduction in both conduction andvalence bands to conduction substantially in one band by one carrieronly. In practice, intrinsic conduction is not completely eliminated butit is reduced to negligible proportions.

[0036] The biasable junction may be an excluding contact for exclusionof minority carriers from the active region, and may be a homojunctionbetween indium antimonide materials of different doping concentration ora heterojunction between indium antimonide and a material having a widerband gap than indium antimonide.

[0037] The sensor may be a cruciform Hall effect sensor with a centralregion from which four limbs extend, at least one limb being connectedto an excluding contact for depleting the active region's minoritycarrier concentration when biased, a first pair of limbs beingconnectable to a current supply and a second pair of limbs beingconnectable to Hall voltage measuring apparatus. Each limb may beconnected to a respective excluding contact, and each limb of the secondpair may have a tapering portion adjacent the central region.

[0038] The biasable junction may be an extracting junction forextraction of minority carriers from the active region. It may be ajunction between two sensor regions of materials having differentmajority carrier type and different band gap, and be sufficiently thickto prevent electron tunnelling and sufficiently thin to avoid relaxingstrain in materials associated with it. It may be heterojunction ofn-type InSb and In_(1-x)Al_(x)Sb where x is in the range 0.10 to 0.5, or0.15 to 0.2, or substantially 0.15.

[0039] The sensor may be cruciform with four limbs and a central regionand have four successively disposed layers of which two adjacent layersare of one majority carrier type and two other adjacent layers are ofthe other majority carrier type, the junction may be a heterojunctionbetween an active region layer and another layer of different band gapand majority carrier type, a first pair of limbs being connectable to acurrent supply, a second pair of limbs (14 b, 14 d) being connectable toHall voltage measuring apparatus, and the junction being an extractingjunction reverse-biasable by means of a sensor substrate connection. Thefour successively disposed layers may be an n⁺npp⁺ structure and thesecond pair of limbs may have a tapering portion adjacent the centralregion. The second pair of limbs may have a contact region adjoining thecentral region which less than 10% of the limb width of each limb of thefirst pair of limbs.

[0040] The junction may be arranged for extraction current flow in adirection substantially orthogonal to charge carrier deflection by amagnetic field in normal operation.

[0041] The active region in the sensor may be p-type and the biasablejunction extracting. It may be provided with a dominant source of chargecarriers in the form of a δ-doping layer. It may incorporate a quantumwell providing a conduction path therein. The sensor itself may be ann⁺-p ⁻-quantum well-p ⁻-p ⁺ diode structure.

[0042] The sensor may alternatively be an n⁺-p-p ⁺-p⁺ diode structure.

[0043] In another aspect, the invention provides a method of detecting amagnetic field, characterised in that it includes the steps of:

[0044] a) providing a magnetic field sensor incorporating asemiconducting sensor element with an active region in which a signalresponsive to magnetic field is developed during operation, the sensorelement being in an at least partially intrinsic conduction regime at anormal operating temperature when unbiased, and including a junctionwhich is biasable to reduce an intrinsic contribution to conduction inthe active region and confine charge carriers predominantly to one typeonly corresponding to an extrinsic saturated regime;

[0045] b) biasing the sensor active region and the junction to providefor charge carrier flow in the active region and sensor operation tocorrespond to an extrinsic saturated regime and applying a magneticfield to the active region; and

[0046] c) detecting a signal developed by the active region at leastpartially in response to the magnetic field.

[0047] The step of biasing the sensor active region may be carried outat constant voltage, the step of detecting a signal involving detectinga voltage signal.

[0048] The sensor may be a Hall effect sensor, the step of biasing thesensor active region involving applying a constant current thereto andthe step of detecting a signal involving detecting a current signal.

[0049] In order that the invention might be more fully understood,embodiments thereof will now be described, by way of example only, withreference to accompanying drawings, in which:

[0050]FIG. 1 is a schematic plan view of a magnetic field sensor of theinvention in the form of a Hall effect device;

[0051]FIG. 2 is sectional view on lines II-II in FIG. 1;

[0052]FIG. 3 shows the energy band structure of the sensor of FIGS. 1and 2;

[0053]FIG. 4 shows the energy band structure of another sensor of theinvention;

[0054]FIG. 5 illustrates a further sensor of the invention;

[0055]FIG. 6 is section on lines VI-VI in FIG. 5;

[0056]FIG. 7 is an energy band diagram for the sensor of FIG. 5;

[0057]FIG. 8 illustrates a central region of the sensor of FIG. 5;

[0058]FIG. 9 gives another geometry for the sensor shown in FIG. 5;

[0059]FIG. 10 illustrates contributions to electron mobility in n-typeInSb under various conditions of sensor operation;

[0060]FIG. 11 illustrates the effect of temperature variation on theHall coefficient R_(H) for equilibrium and extracted InSb;

[0061]FIG. 12 is an illustration of the variation of Hall coefficientR_(H) with magnetic flux density for both equilibrium and extractedInSb;

[0062]FIG. 13 is a circuit for the sensor of FIG. 1;

[0063]FIG. 14 is a circuit for the sensor of FIG. 5.

[0064]FIG. 15 is a sectional view of a magnetic field sensor of theinvention in the form of a magneto-resistive device; and

[0065]FIG. 16 is a sectional view of a magneto-resistive sensor of theinvention incorporating a quantum well;

[0066] Referring to FIGS. 1 and 2, there is shown a Hall effect magneticfield sensor 10 of the invention in plan and section respectively. Itincorporates a cruciform layer 12 of n-type indium antimonide (InSb)with four limbs 14 a to 14 d (collectively 14) extending from a squarecentral region 14 e, limbs 14 a and 14 c being orthogonal to limbs 14 band 14 d. Distal end lengths of the limbs 14 are covered by regions 16 ato 16 d (collectively 16) each consisting of a respective n⁺-type InSblayer 17 a to 17 d (collectively 17) surmounted by aluminium (Al)electrodes 18 a to 18 d (collectively 18) respectively. The superscript“+” in n⁺ denotes much higher doping concentration than that in thelayer 12.

[0067] The sensor 10 has an insulating substrate 20 of sapphire, highresistivity silicon (>50 ohms/square) or insulating GaAs. Asillustrated, the InSb layer 12 is attached to the substrate 20 by glue22, but may alternatively be grown directly on a substrate such assemi-insulating GaAs. Each of the four electrodes 18 makes an ohmiccontact to its respective n⁺-type InSb layer 17 and is bonded to arespective connection wire 19.

[0068]FIG. 3 is an energy band structure diagram 30 for the n and n⁺type InSb layers 12 and 17. It shows conduction and valence bands 32 and34 for an unbiased n⁺n junction 36 between the layers 17 and 12 havingparts 36 b and 36 d in FIG. 2.

[0069] The sensor 10 operates as follows. It is a minority carrierexclusion device in which each n⁺-type layer 17 forms an n⁺nhomojunction 36 with the underlying n-type layer 12. As previouslyindicated, carrier exclusion is known per se in relation to photodiodesfrom U.S. Pat. No. 5,016,073 to Elliott and Ashley. A bias voltage isapplied across contacts 18 a and 18 c on mutually opposite limbs 14 aand 14 c, contact 18 a being positive relative to contact 18 c. Contacts18 b and 18 d on mutually opposite limbs 14 b and 14 d are voltageprobes for measuring Hall voltage. Because the n⁺-type layer 17 a isheavily doped it has a negligible minority carrier (hole) concentration.It is therefore capable of accepting majority carriers (electrons) fromthe layer 12 but largely incapable of returning to it minority carriers(holes) in view of its dearth of the latter. The n⁺n homojunction 17a/12 or 36 is therefore an excluding contact, that is electrons(majority carriers) flow freely from layer 12 to layer 17 a but only amuch smaller hole (minority carrier) current flows in the reversedirection from layer 17 a to layer 12.

[0070] Moreover, holes are removed from the layer 12 at the opposite n⁺nhomojunction between layers 17 c and 12. In consequence, application ofa bias voltage across contacts 18 a and 18 c results in the minoritycarrier concentration in the layer 12 becoming depleted because holesare removed from it but not fully replenished. The majority carrierconcentration in this layer must fall by the same amount as the minoritycarrier concentration from charge neutrality considerations; electronsand holes are therefore reduced in equal numbers, which reduces theintrinsic contribution to conduction in the layer 12 (a reduction in theextrinsic contribution would affect the majority carrier type only). Theexcluded region, ie the region which is depleted of charge carriers inthis way, extends through the layer 12 in limbs 14 b and 14 d betweenn⁺-type layers 17 a and 17 c.

[0071] Chromium-gold (Cr—Au) electrodes may be employed instead of Alelectrodes 18 by depositing a seed layer of Cr on the n⁺-type layers 17and then a thicker Au layer thereon.

[0072] As indicated at 39A to 39C, in which parts previously describedare like-referenced, each n⁺-type InSb layer 17 may be replaced by alayer 40 of n-type material, or alternatively by two layers, an n layer41 and an n⁺ layer 42; here underlining in n (and later in p also)indicates a wider band gap material than a non-underlined equivalent, ien. FIG. 4 is a band structure diagram showing the consequences ofreplacement of layer 17 by a layer 40 of n-type material ofIn_(1-x)Al_(x)Sb with x=0.15. Conduction and valence bands 43 and 44 areshown for an unbiased nn heterojunction 46 formed between the n-typelayer 40 and the n-type InSb layer 12.

[0073] An n layer 40 has a low minority carrier (hole) concentration,because its wide band gap results in fewer electron-hole pairs beingthermally excited than in a narrower band gap material. A bias voltageapplied across contacts 18 a and 18 c removes holes from the n-typelayer 12 which cannot be replaced from the n-type layer 40 because ofits lack in this regard. Similar remarks apply to the layers 41 and 42.

[0074] Referring now to FIG. 5, there is shown another Hall effectsensor 50 of the invention. It incorporates a cruciform structure 52 ofn-type indium antimonide with four limbs 53 a to 53 d extending from asquare central region 53 e (collectively 53), limbs 53 a and 53 c beingorthogonal to limbs 53 b and 53 d. Distal end lengths of the limbs 53are covered by respective n⁺-type InSb layers 54 a to 54 d (collectively54) surmounted by aluminium electrodes (not shown). Dimensions of thelimbs 53 and central region 53 e are indicated by parameters a, b and c,where:

[0075] a=length of limb 53+side of square 53 e

[0076] b=length of limb 53

[0077] c=a−b=width of limb 53=side of square 53 e

[0078]FIG. 6 is a section on line VI-VI in FIG. 5 perpendicular to theplane of the latter and showing the sensor 50 layer structure. It is notdrawn to scale. The sensor 50 comprises a 2 μm thick layer 64 of p⁺-typeInSb upon a substrate 62 of InSb or GaAs. The layer 64 is surmounted bythe cruciform structure 52 which comprises a 20 nm thick layer 66 of p⁺-type In_(1-x)Al_(x)Sb with x in the range 0.1 to 0.5, preferably 0.1to 0.2, eg 0.15. The layer 66 is covered by a 0.5 μm thick layer 53 ofn-type InSb from which the limbs and centre 53 are constituted.Electrode layers 54 b and 54 d on end lengths of respective limbs 53 aren⁺-type TnSb 0.3 μm thick; The n⁺-type layers 54, the n-type layer 53and p ⁺-type layer 66 each have two end faces lying in respective planes74 and 76 and corresponding to end faces of limbs 53. The end faces 74and 76 adjoin field insulators 78 fabricated on the p⁺-type InSb layer64. Alternatively, a layer of polyamide may be used instead of fieldinsulator. Al contact layers 80 and 82 are formed on each n⁺-type InSbregion 54 and its adjacent field insulator 78. The substrate 62 has afifth contact 84 is ohmic and of aluminium.

[0079]FIG. 7 provides the band structure of the sensor 50 in the absenceof any applied bias, and comprises a conduction band 102, a valence band104 and a Fermi Level 106. The n⁺-type layers 54 form four n⁺nhomojunctions with the n-type limbs 53 at first interfaces 108; then-type limbs and centre 53 form a np heterojunction with the p ⁺ -typelayer 66 at a second interface 110; the p ⁺-type layer 66 forms a p ⁺-p⁺ heterojunction with the p⁺-type layer 64 at a third interface 112.

[0080] The p ⁺ -type layer 66 forms a barrier in the conduction band 102inhibiting electron flow from the p⁺-type layer 64 to the n-type andn+-type layers 53 and 54.

[0081] The sensor 50 operates as follows. The semiconductor layers 64,66, 53 and 54 form four n⁺npp⁺ diode structures, of which two are shownin section in FIG. 6. The layers 64, 66 and 53 and np junction 110 arecommon to all four diodes, but each has a separate layer 54 as shown inFIG. 5. The np junction 110 is reversed biased by applying a voltagebetween the substrate electrode 84 and one of the surface electrodes 80or 82 This has the important effect of extracting minority carriers fromthe n-type region 53.

[0082] The principle of carrier extraction is known in the prior art andis described in for example European Pat No EP 0167305 and U.S. Pat. No.5,016,073. It consists of removing minority carriers from asemiconductor region at a greater rate than they are replaced; thisoccurs at a biased pn junction to which minority carriers diffuse and atwhich they are extracted and become lost to the region. Carriertransport across the np junction interface 110 comprises:

[0083] (a) a conduction current of majority carriers possessingsufficient thermal energy to surmount the junction potential barrier;and

[0084] (b) a diffusion current of minority carriers which diffuse to thejunction and are swept over it by its potential drop.

[0085] Thus, carrier transport across the np junction interfaces 110,112 comprises:

[0086] (a) a hole conduction current from the p⁺-type region 64 to then-type region 53;

[0087] (b) an electron diffusion current from the p⁺⁻type region 64 tothe n-type region 53—this is very small because there are few minorityelectrons in p⁺ and p

[0088] (c) an electron conduction current from the n-type region 53 tothe p⁺-type region 64 which is also very small; and

[0089] (d) a hole diffusion current from the n-type region 53 to thep⁺-type region 64.

[0090] In the following qualitative description of operation negligiblysmall electron currents are ignored.

[0091] In the absence of bias, conduction and diffusion hole andelectron currents across each interface 108, 110, 112 are balanced,namely the sensor 50 is in equilibrium. A bias voltage is applied acrosseach of the diode structures so that the np junction at the interface110 is reverse biased; in consequence, minority carriers (holes) in then-type cruciform layer 53 which diffuse to the interface 110 are sweptacross it by its potential drop. At the same time, this potential dropinhibits flow of holes from layer 66 to the cruciform layer 53. Thereverse bias therefore substantially decreases both the electronconduction current from the cruciform layer 53 to the p⁺-type region 64,and the hole conduction current from the p⁺-type region 64 to thecruciform layer 53. Holes are therefore removed from the n-type region53 by diffusion, and cannot be fully replaced by conduction from thep⁺-region 64 because of the potential barrier of the reverse biased npjunction at the interface 110. As indicated earlier this is the minoritycarrier extraction effect.

[0092] The cruciform n-type layer 53 receives a negligibly small hole(minority carrier) current from the n⁺ layers 54 because their holeconcentrations are very small: this is the minority carrier exclusioneffect. In consequence, holes in the layer 53 diffusing to the interface110 and extracted to the p ⁺-type layer 66 cannot be adequatelyreplenished from the n⁺ layers 54, and therefore the minority carrierconcentration in the cruciform layer 53 is reduced. As describedearlier, from charge neutrality considerations, the majority carrierconcentration must fall to the same extent as the minorityconcentration, ie electron and hole concentrations fall equallycorresponding to reduction in electron-hole pairs: this thereforereduces the intrinsic contribution to conduction in the cruciform layer53.

[0093] In operation of the sensor 50, the np junction 110 reversebiased, and the sensor 10 functions with bias current flowing between anopposite pair of contacts 16 a/16 c, the positive contact beingexcluding. Both sensors 10 and 50 operate with their n-type layers 12and 53 having below-equilibrium concentrations of electron-hole pairs;these layers are the active regions for magnetic field measurements, andHall voltages are detectable across pairs of contact layers 16 b/16 dand 54 b/54 d. In the sensor 10, carrier concentration is aboveequilibrium near a negatively biased contact because of carrieraccumulation.

[0094]FIG. 8 is a three-dimensional view of the central square section53 e of the sensor 50 together with supporting p⁺-type layer 64 andsubstrate 62. Limbs 53 are indicated by dotted lines. Cartesian axes areshown at 122 for use in defining magnetic field and current flowdirections. The surface of the central square section 53 e is in the XYplane, has pairs of sides parallel to the X and Y axes respectively, andis perpendicular to the Z axis. Current through the sensor 50 has firstand second components I_(plane) and I_(⊥) indicated by arrows 124 a and124 b. The first current component I_(plane) flows parallel to the Xaxis substantially within the n-type layer 53 and between opposite limbs53 a and 53 c.

[0095] The second current component I_(⊥) flows parallel to the Z axisand results from biasing the np junction interface 110. It does nottherefore, strictly speaking, flow within the section 53 e, but consistsof four contributions flowing to respective end layers 54 on the limbs53 and is included in FIG. 8 for ease of reference. It arises fromthermal charge carrier generation in the n-type layer 53, and thereforethe associated flow of holes is essentially uniform in a directionparallel to the Z axis down to the p ⁺-type layer 66, whilst electronflow occurs laterally and upwardly to the n⁺ layers 54 for electronsoriginating in the n-type cruciform layer 53 and not directly under then⁺ regions 54.

[0096] A voltage is applied between regions 54 a and 54 c to establishthe current flow I_(plane) which corresponds to a current density j_(x)flowing parallel to the X axis. Referring once more to FIG. 6, it can beseen that the sensor 50 experiences both extraction and conductionelectric fields. The extraction field is applied between the substrateelectrode 84 and each of the four limb end layers 54 a to 54 drespectively. The conduction field is applied between the first pair ofopposed limb end layers 54 a and 54 c. The extraction field gives riseto the current component I_(⊥), which is a diode leakage parallel to theZ axis. The conduction field produces the current component I_(plane)having current density j_(x), which is predominantly electron flow inthe n-type layer 53 because of the np interface 110 acting as a block tofield penetration of the p-type layer 64. The sensor 50 is in a magneticfield B_(z) parallel to the Z axis and therefore orthogonal to the planeof FIG. 5. The current I_(plane) and magnetic field B_(z) develop a Hallvoltage in the central region 53 e parallel to the Y axis, the voltagebeing measurable between the second pair of limb end layers 54 b, 54 d.

[0097] The current I_(plane) is substantially confined to the n-typelayer 53 and the Hall voltage is accordingly developed within thislayer. However, by virtue of the carrier extracting and excludingproperties of the n⁺npp⁺ structures in the sensor 50, the intrinsiccontribution to the carrier concentration is reduced as described above.Conduction and Hall effect are predominantly due to extrinsic conductionin a saturated extrinsic regime with a carrier concentration largelyindependent of temperature. Moreover, extraction reduces the carrierconcentration which reduces electron-hole scattering and its effect oncarrier mobility, which in turn becomes less sensitive to temperaturechange.

[0098] Referring to FIG. 9, an alternative form of sensor 128 is shown.It is as in the sensor 50 (like parts being like-referenced) except thatit has Hall voltage sensing limbs 53 b and 53 d which taper to a width dadjacent the central section 53 e. Where untapered, all limbs 53 are ofwidth c, and d is less than c; d is preferably less than one tenth of c.

[0099] In operation of the sensor 50, minority carriers are extractedfrom the central section 53 e region. The bias field giving rise to thecurrent I_(⊥) must therefore extract charge carriers not only from theends of the limbs 53 a to 53 d but also from the n-type layer 53throughout the central region 53 e. The length of each limb 53 a to 53 dtogether with the central node 53 e, namely dimension a, must besufficiently short to allow extraction to extend throughout the limbs 53a to 53 d and the central node 53 e. However, the path length over whichthe Hall effect is experienced must be large enough to allow chargecarriers to be deflected and generate a measurable signal.

[0100] A constraint with regard to the sensor 10 is a need to avoidcharge accumulation within the central region 14 e. When limb end layers16 a and 16 c are biased, charge accumulates at whichever of these isnegatively biased with respect to the other. Accumulation must besufficiently far from the central region 14 e to ensure that it does notlessen the carrier exclusion. This therefore provides a minimum lengthfor each limb 14 a to 14 d (dimension b in FIG. 5) which is governed bysensor operating conditions and limited by an upstream diffusion lengthL_(d) given by: $\begin{matrix}{L_{d} \approx \left\{ {\frac{1}{2}\left\lbrack {\frac{- {qE}}{kT} - \sqrt{\frac{qE}{kT} + \frac{4}{l^{2}}}} \right\rbrack} \right\}^{- 1}} & (6)\end{matrix}$

[0101] where

[0102] E=applied electric field;

[0103] q=carrier charge;

[0104] k=Boltzmann's constant;

[0105] T=absolute sensor temperature; and

[0106] I=zero field carrier diffusion length.

[0107] The zero field carrier diffusion length l is given by:

l={square root}{square root over (D)}τ  (7)

[0108] where

[0109] D=charge carrier diffusion coefficient; and

[0110] τ=charge carrier mean lifetime.

[0111] In the sensor 50, the Hall current I_(plane) is affected by thediode leakage current I_(⊥). To reduce this, I_(plane) is preferablymuch greater than I_(⊥). However, I_(⊥) is dependent on bias voltageV_(bias) across the n⁺npp⁺ structure of the sensor 50, and V_(bias) mustbe large enough to ensure effective extraction. Alternatively, I_(plane)may be made as large as possible within limits set by the power densitythe sensor 50 can tolerate. This implies a small cross-sectional areathrough which I_(plane) flows, and is achievable by reducing the n-layerheight t_(z) and width c. The minimum height of the n-type layer 53 isdetermined by the width of depletion region it must support. This varieswith doping level and bias voltage magnitude. Thus, for a given dopinglevel and bias strength, the only remaining variable parameter is thewidth c of the side of the cruciform structure 53. Suitable values of cwill be discussed later.

[0112] Similar remarks apply to the sensor 10, for which the biasvoltage needs to large enough to ensure adequate carrier exclusion.

[0113] The sensors 10 and 50 show performance improvements compared toprior art equilibrium devices, as shown graphically in FIGS. 10, 11 and12. These drawings are based on calculations involving an n-type InSblayer 53 with donor impurity concentration of 10¹⁶ cm⁻³. They includethe effects of both electron and hole contributions to the Hall effectand are therefore more complicated than approximations given earlier.The carrier concentration and therefore also the Hall coefficient in thesensors 10 and 50 is not totally temperature independent, but theirvariation is sufficiently low (˜30-40% change over 50K) for a number ofapplications. Examples described later improve on this.

[0114] In a sensor where carriers from more than one band, ie bothelectrons and holes, contribute to conductivity, expressions for theHall voltage V_(H) are more complicated and are magnetic fielddependent. See eg the standard textbook “The Hall Effect andSemiconductor Physics”, E H Putley, published by Butterworth and Co.,1960, Chapter 4. The magnetic field dependence is more marked inmaterials with higher mobilities than silicon.

[0115] From Equations (2) and (3), substituting for current density$j_{x} = {\frac{I_{x}}{t_{y}t_{z}}: -}$

[0116] where

[0117] t_(z)=sensor thickness dimension parallel to magnetic field,

[0118] I_(x)=longitudinal sensor current flow orthogonal to Hall andmagnetic fields, and other parameters are defined earlier.$\begin{matrix}{V_{H} = \frac{R_{H}{BI}_{x}}{t_{z}}} & (8)\end{matrix}$

[0119]FIG. 10 provides four curves 132 to 138 of electron mobility μ_(e)plotted against temperature for n-type InSb for different scatteringmechanisms and conditions of operation. It illustrates the temperaturesensitivity of Hall coefficient R_(H) for narrow-gap semiconductors whenboth electrons and holes contribute to conduction. The first curve 132corresponds to mobility being affected only by scattering from ionisedimpurities and from electron-hole interactions. The second curve 134indicates the temperature variation of that component of mobility whichis affected solely by optical phonon scattering. The third curve 136indicates carrier mobility arising from the sum of the contributionsgiving rise to the first and second curves 132, 134. These three curves132, 134 and 136, were produced on the basis of an equilibrium carrierconcentration. The fourth curve 138 is the variation of mobility μ_(e)with temperature T when the intrinsic contribution to conduction hasbeen extracted.

[0120] Comparison of curves 136 and 138 illustrates the beneficialeffect of carrier extraction for sensors according to the invention,because extraction increases mobility at temperatures above about 250K:differences between the equilibrium curve 136 and the extracted curve138 become more pronounced at higher temperatures leading to larger Hallcoefficient and magneto-resistance. Comparison of the gradients of thesetwo curves shows that the variation of mobility with temperature T isalso slightly reduced by extraction. This lessens the temperaturedependence of Hall coefficient and magneto-resistance.

[0121]FIG. 11 provides two curves 142 and 144 of Hall coefficient R_(H)plotted against temperature for an InSb semiconductor in an 0.3Tmagnetic field under equilibrium and extraction conditions respectively.In the equilibrium curve 142, R_(H) falls by something approaching twoorders of magnitude in the interval between 150K and 500K. The secondcurve 144 is the variation in R_(H) with temperature for a sensor 50undergoing minority carrier extraction in accordance to the invention;here R_(H) is substantially independent of temperature in the sameinterval indicating the superiority of sensors of the invention withrespect to temperature insensitivity.

[0122]FIG. 12 shows four curves 152, 154, 156 and 158 of R_(H) plottedagainst applied magnetic field for both equilibrium and extracted InSbunder various temperature conditions. It illustrates the magnetic fieldsensitivity of R_(H) for narrow-gap semiconductors in an intrinsicregime when both electrons and holes contribute to conduction. Curve 152is for an extracted sensor of the invention, and shows R_(H) is at leastsubstantially independent of magnetic field. Curve 154 is for a sensorunder equilibrium conditions at 200K, and shows R_(H) is only a littledependent on field—falling by ˜3% between 0.1T and 1.5T. Curves 156 and158 are for a sensor under equilibrium conditions at 300K and 400Krespectively; these show R_(H) reversing in sign and between 0.1T and1.5T dropping from +200 cm³/C. to −10 cm³/C. in one case and +30 cm³/C.to −50 cm³/C. in the other. This indicates the superiority of sensors ofthe invention with regard to magnetic field effects.

[0123] Parameters of sensors of the invention which affect operation areas follows:

[0124] (a) Sensor operating temperature range: sensor current densityincreases with operating temperature (eg 370K), which may give rise tocharge carriers with sufficient energy to surmount the barrier at theinterface 110;

[0125] (b) Composition of In_(1-x)Al_(x)Sb barrier layer 66: Table 1below gives leakage current density as a function of sensor operatingtemperature for a range of barrier materials (x values) and donorconcentrations N_(d);

[0126] (c) Electrical currents: diode leakage current I_(⊥) ispreferably 1% of the Hall current I_(plane), although acceptable sensormeasurement accuracy may be obtained with I_(⊥)˜10% I_(plane).

[0127] (d) Doping concentration of the n-type layer 53: this limitsmaximum current.

[0128] (e) Power density: this has to be limited to a sustainable levelwithin the layer 53 to avoid thermal runaway, eg ˜100 W cm⁻². For thesensor 50, the power density P_(d) is given by: $\begin{matrix}{P_{d} = \frac{I_{plane}^{2}}{\left( {{en}\quad \mu \quad l^{2}t_{z}} \right)}} & (9)\end{matrix}$

[0129]  where

[0130] I_(plane)=current flowing in the plane of the layer 53;

[0131] e=carrier charge;

[0132] n=carrier concentration;

[0133] μ=carrier mobility in the layer 53;

[0134] l=sensor current path length; and

[0135] t_(z)=thickness of the layer 53.

[0136] (i) Applied voltages: in addition to the Hall voltage V_(H), twoother voltages are associated with the sensor 50: a voltage V_(bias) (eg0.5V) between the substrate electrode 84 and a limb electrode 80 or 82reverse biases the np junction 110; a voltage V_(drive) between oppositelimbs 53 a and 53 c drives the current I_(plane). The voltage V_(bias)extracts thermally generated charge carriers from the n-type region andit affects the thickness of the depletion layer of the extracting pnjunction.

[0137] (g) The thickness t_(z) of the n-type layer 53 should besufficient to support the pn depletion layer: for doping of 10¹⁶ cm⁻³and V_(bias) of 1V it is preferably 0.5 μm.

[0138] (h) The thickness of the p ⁺-type layer 66 is preferably 20 nm.This layer provides a barrier ˜10 nm or more thick sufficient to preventelectron tunnelling. The barrier is also sufficiently thin (<30 nm) topreserve strain between it and adjoining InSb layers.

[0139] A theoretical model for the sensor 50 has been used to identifysuitable device parameters which appear in Tables 1, 2 and 3.

[0140] Table 1 lists leakage current densities j_(⊥) as a function ofabsolute temperature T for a variety of different doping levels N_(d)and composition parameters x for In_(1-x)Al_(x)Sb. TABLE 1 j_(⊥) (Acm⁻²) x = 0.15 x = 0.15 x = 0.15 x = 0.20 x = 0.25 T(K) N_(d) = 10¹⁷cm⁻³ N_(d) = 10¹⁶ cm⁻³ N_(d) = 10¹⁵ cm⁻³ N_(d) = 10¹⁵ cm⁻³ N_(d) = 10¹⁵cm⁻³ 230 0.24 0.073 0.23 0.23 0.23 250 0.85 0.25 0.61 0.61 0.61 270 2.540.75 1.38 1.36 1.36 290 6.77 1.98 2.79 2.66 2.65 310 16.5 4.84 5.4 4.734.7 330 37.9 11.5 10.8 7.9 7.73 350 82.9 27.5 23.3 12.7 12 370 176 66.254.5 20.8 18 390 365 158 132 35.8 26.5 410 739 365 314 67.5 39.3 4301456 803 711 138 60.6 450 2772 1666 1507 291 99.8

[0141] For values for x and N_(d) in Table 1, Table 2 gives other sensorparameters for an operating temperature of 370K and leakage current toin-plane current ratio I_(⊥)/I_(plane) limited to 0.9% to 1.1%. TABLE 2(T = 370 K; I⊥/I_(plane)˜1%) Nd I_(plane) t_(z) l V_(bias) P_(d) (cm⁻³)x (mA) (μm) (μm) (volts) (W/cm²) I_(⊥)/I_(plane) 10¹⁷ 0.15 1.0 0.2 6.30.03 78 0.009 10¹⁶ 0.15 0.5 0.5 5.0 0.06 124 0.011 10¹⁵ 0.2 0.2 1.5 3.20.08 166 0.010

[0142] Values in Table 3 are equivalent to those of Table 2 except thatthe ratio I_(⊥)/I_(plane) is increased to a range of 10% to 12%. Thesensors 10 and 50 may have I_(⊥)/I_(plane) in the range 1% to 10%. TABLE3 (T = 370 K; I_(⊥)I_(plane)˜11%) Nd I_(plane) t_(z) l V_(bias) P_(d)(cm⁻³) x (mA) (μm) (μm) (volts) (Wcm⁻²) I_(⊥)/I_(plane) 10¹⁷ 0.15 2.00.2 100 0.62 124 0.12 10¹⁶ 0.15 2.0 0.5 32 0.25 50 0.11 10¹⁵ 0.2 1.0 1.522 0.42 83 0.10

[0143] Table 1 indicates the difficulty of maintaining a leakage currentdensity j_(⊥) at a reasonable level at higher operating temperatures T.Increasing the composition parameter x of the barrier layer 66 reducesthe leakage current density j_(⊥) and the drive current necessary tomaintain a constant ratio I_(⊥)/I_(plane); it also reduces sensor powerdissipation. For example, a change in composition in x from 0.15 to 0.25allows larger current densities to be supported and means that leakagecurrent density of 55 Amp/cm² for N_(d)=10¹⁵ cm⁻³ corresponds to anoperating temperature of approximately 200K, as opposed to 370K.

[0144] Table 2 shows that, when an impurity concentration of 10¹⁵ cm⁻³is employed, it becomes difficult to find conditions under which thepower density P_(d) remains reasonable for reliable sensor operation,even with a larger barrier composition. On the other hand, increasingimpurity concentration from ˜10¹⁵ cm⁻³ to ˜10¹⁷ cm⁻³ reduces carriermobility by a factor of ˜3. Moreover, the proximity of highly n-typeregions to p-type regions can result in carrier tunnelling contributingto j_(⊥). Table 2 therefore indicates that an optimum dopingconcentration is ˜10¹⁶ cm⁻³; a sensor current path length of 5 μm givesan acceptable power density P_(d) of 124 watt/cm².

[0145] Table 3 indicates that increasing I_(⊥)/I_(plane) allows largersensors to be used: the latter are easier to make and support largercurrents for equivalent power density giving larger Hall voltages andbetter sensitivity.

[0146] Referring now to FIG. 13, there is shown a circuit 200 for thesensor 10. A battery 210 with positive and negative terminals 212 and214 is connected directly to limb end layer 16 c and through a seriesresistor R_(L) to limb end layer 16 a respectively.

[0147] The battery 210 biases end layer 16 c positive with respect toend layer 16 a, and provides a current I_(plane) through end layer 16 c,limb 14 c, central region 14 e, limb 14 a and end layer 16 c. Because ofits positive bias, end layer 16 c is an excluding contact to the n-typeactive region 14, which in consequence becomes depleted of equal numbersof electrons and holes as described earlier largely eradicating theintrinsic contribution to conduction. The exclusion zone extends throughthe limb 14 c, central region 14 e and limb 14 a. With a magnetic fieldapplied normal to the plane of the drawing, a Hall voltage is developedbetween the regions 16 b, 16 d. The current flowing between end layer 16c and 16 a is predominantly due to one carrier type only, ie electronsactivated from donor impurities, and the sensor operates in a regimewhich simulates the extrinsic saturated regime of a wider band gapmaterial such as Si.

[0148] Referring now to FIG. 14, there is shown a circuit 300 for thesensor 50. The circuit 300 has a first battery 310 with positive andnegative terminals 312 and 314 connected directly to limb end layer 16 cand through a series resistor R_(S) to limb end layer 16 a respectively.A second battery 320 has a negative terminal 322 connected through aseries resistor R_(B) to a sensor substrate connection 330, and also apositive terminal 334 connected to region 16 c and to the firstbattery's negative terminal 314.

[0149] The first battery 310 biases the sensor 50 through resistorR_(S), and current I_(plane) flows between end layers 54 a and 54 c vialimb 53 a, central region 53 e and limb 53 c. The second battery 330biases the substrate 62 (see FIG. 6) relative to end layers 54 a and 54c, which reverse biases the np heterojunction 110 between layers 53 and66. Layer 66 acts as an extracting contact to layer 53, in which theintrinsic contribution to conduction is largely eradicated inconsequence. The current I_(plane) in the layer 53 is thereforepredominantly due to one carrier type only, ie electrons activated fromdonor impurities, and the sensor 50 operates in a regime which simulatesthe extrinsic saturated regime. With a magnetic field normal to theplane of the drawing, the sensor 50 develops a Hall voltage V_(H) acrossthe central region 53 e detectable across the end layers 54 b and 54 d.

[0150] Hall effect and magneto-resistance sensors are normally operatedin a current driven mode in which sensor current is held constant andchange in voltage is detected to indicate magnetic field B, which for aHall effect sensor is given by: $\begin{matrix}{B = \frac{V_{H}{ent}_{z}}{I_{x}}} & (10)\end{matrix}$

[0151] where parameters are as defined earlier.

[0152] From Equation (1):

R _(M) =R ₀(1+μ² B ²)=V _(D) /I _(x)  (11)

[0153] where V_(D) is longitudinal voltage driving a current I_(x)through a magneto-resistance sensor; rearranging: $\begin{matrix}{B = \frac{l_{x}V_{H}}{t_{y}\mu \quad V_{D}}} & (13)\end{matrix}$

[0154] In current driven mode the measured value of B depends on thecarrier concentration n, which is subject to generation—recombinationnoise in the semiconductor and affects the measurement also. Sensorswith charge carrier extraction are influenced by 1/f noise due either tocarrier concentration fluctuation or to mobility fluctuation. Existing(inconclusive) evidence favours concentration fluctuation: if so thecurrent driven/voltage read mode would be subject to 1/f noise.

[0155] An alternative mode of operation for a magnetic field sensor ofthe invention is voltage drive—ie operation at constant drivevoltage—and voltage read to indicate magnetic field B: for amagneto-resistance sensor B remains given by Equation (12), and for aHall effect sensor it is given by: $\begin{matrix}{B = {\frac{1}{\mu}\sqrt{\frac{V_{D}}{I_{x}R_{0}} - 1}}} & (12)\end{matrix}$

[0156] here l_(x) is sensor length and other terms are as definedearlier. In voltage drive mode the measured value of B is independent ofcarrier density and is temperature dependent only because of mobilitytemperature dependence: the latter is a slow variation and alsocounteracts effects due to any residual carrier concentration changebecause the two produce opposite effects. Moreover, this measured valueof B would not be subject to generation—recombination noise or 1/fnoise, if the latter is due to density fluctuations.

[0157] Voltage drive mode is not generally used because it can causesensor thermal runaway and instability. However, a sensor of theinvention is stabilised against thermal runaway because the latter isdue to intrinsic conduction which the invention reduces. Furthermore,operation in this mode does not require reduction of the intrinsiccontribution to as great a degree as other modes to obtain equivalentperformance. It is also expected to result in operability of a sensorover a larger temperature range than current driven mode.

[0158] A further alternative is to operate a Hall effect sensor of theinvention in a current drive and current read mode: in this mode sensorcurrent I_(x) is held constant and Hall voltage is employed to drive acurrent in a external circuit connected across Hall voltage electrodes,and the latter current is measured. This mode should in principle havethe same advantages as the voltage drive and voltage read mode. Currentflow pattern in a current drive and current read mode through such asensor is complex and requires numerical modelling for full assessment.Current read out is used when there is a need to drive a device directlyusing the sensor output signal.

[0159] Referring now to FIG. 15, a magneto-resistive sensor of theinvention 400 is shown in section, but as indicated by zigzag lines suchas 402 is not drawn to scale. Parts equivalent to those shown in FIG. 6are like-referenced with a prefix 400. The sensor 400 includes a 1 μmthick substrate layer 464 of p⁺-type InSb upon a substrate 462 of InSbor GaAs and having an electrical bias contact 484, which may berelocated more remotely if desired. The layer 464 is surmounted by a 20mn thick layer 466 of p ⁺-type In_(1-x)Al_(x)Sb with x in the range 0.1to 0.5, preferably 0.1 to 0.2, eg 0.15. The layer 466 is covered by a0.5 μm thick layer 404 consisting largely of p-type InSb with a dopantconcentration of 3×10¹⁵ cm⁻³, but incorporating 30 nm below its surface406 an ultra-thin layer of silicon 408 indicated by a chain line: thesilicon layer 408 is referred to as a δ-doping layer. In operation theδ-doping layer 408 provides a two dimensional electron gas withconcentration in the range 6×10¹¹ cm⁻² to 2×10¹² cm⁻², e.g. 1×10¹² cm⁻².

[0160] Two n+ regions 411 of InSb 30 nm thick are deposited on the layer404 providing electrical connections to it: they allow voltage acrossthe sensor 400 to be measured and hence sensor resistance determined toprovide a measure of magnetic field. They are separated by a distance inthe range 2 to 5 μm, eg 3.5 μm. In plan, the sensor 400 is as shown inFIG. 1 except there are no limbs 14 a and 14 c.

[0161] The sensor 400 is a n⁺-p-p ⁺-p⁺ diode structure in which the player 404 undergoes carrier extraction when a reverse bias is applied,ie when one of the electrodes 411 is biased positive with respect to thesubstrate 462. This is because the interface between layers 411 and 404is an n⁺p junction which is an extracting contact when reverse biased.The carrier concentration is reduced to well below the intrinsicconcentration prevailing during absence of bias, and it becomes largelyindependent of temperature as in the saturated extrinsic regime.

[0162] A conducting layer of electrons is provided to the p layer 404 bythe δ-doping layer 408: the electrons form a two-dimensional gas with aconcentration which also remains largely constant with temperature,because it is set by a doping concentration, not by thermal activation.Electrons from the δ-doping layer 408A are the dominant source of chargecarriers in the p layer 404, which is the sensor active region. The n⁺layers 411 act as source and drain connections to the p layer 404, whichprovides the conducting path between them. It is the resistance of thisconducting path that is magnetic field dependent according to Equation(1), and provides the magneto-resistance effect by which magnetic fieldis measured.

[0163] The minority carrier (electron) mobility and hence also theelectron diffusion length are much higher in a p-type semiconductormaterial than the hole mobility in an n-type equivalent: the carrierextraction effect extends over a minority carrier diffusion length, andin consequence of the two conductivity types p-type material undergoesmuch more efficient extraction, and the carrier concentration has agreater degree of temperature independence. In the sensor 400 thecarrier concentration and resistance R₀ change over 50K is about 2%,which is sufficiently constant for many applications.

[0164] Referring now to FIG. 16, a magneto-resistive sensor 500 is shownin section, but as indicated by zigzag lines such as 502 is not drawn toscale. The sensor 500 comprises a 1 μm thick layer 504 of p ⁺-typeIn_(0.85)Al_(0.15)Sb with a dopant concentration of 2×10¹⁸ cm⁻³. Thelayer 504 is upon a substrate 506 of InSb or GaAs and has an electricalbias contact 508, which may be more remotely located. The layer 504bears an 0.5 μm thick layer 510 of p⁻-type In_(0.85)Al_(0.15)Sb which isnominally undoped—less than 1×10¹⁶ cm⁻³. The layer 510 is covered by a15 nm thick quantum well 512 of p-type InSb with a dopant concentrationof 3×10¹⁵ cm⁻³. The quantum well 512 is covered by a 150 nm thick layer514 (acceptable thickness range 100-200 nm) consisting largely of p⁻-type In_(0.85)Al_(0.15)Sb which is nominally undoped—less than 1×10¹⁶cm⁻³. The p ⁻-type layer 514 incorporates a silicon n-type δ-dopinglayer 518 above the quantum well 512 and spaced apart from it by adistance in the range 10-40 nm. In operation the δ-doping layer 518provides a two-dimensional electron gas with concentration in the range6×10¹¹ cm⁻² to 2×10¹² cm⁻², eg 1×10¹² cm⁻² which forms in the quantumwell 512 because it is energetically favourable: this is referred to asmodulation doping and the electron gas concentration also remainsconstant with temperature.

[0165] Two n⁺ regions 520 of InSb 30 nm thick are deposited on andprovide electrical connections to the layer 514: they allow voltageacross the sensor 500 to be measured and hence sensor resistancedetermined to provide a measure of magnetic field. They are separated bya distance in the range 2 to 5 μm, eg 3.5 μm. In plan, the sensor 500 isas shown in FIG. 1 except that limbs 14 a and 14 c are absent.

[0166] The sensor 500 is a n⁺-p ⁻-quantum well-p ⁻-p ⁺ diode structurein which the quantum well 512 undergoes carrier extraction when areverse bias is applied, ie with one or both of the electrodes 520biased positive with respect to the substrate 506. This is because theinterface between layers 514 and 520 is an n⁺p junction which is anextracting contact when reverse biased. The carrier concentration in thequantum well 512 is reduced to well below the intrinsic equivalent forabsence of bias, and here again it becomes largely independent oftemperature as in a saturated extrinsic regime. Electrons from theδ-doping layer 408 are then the dominant source of charge carriers inthe quantum well 512 which is the sensor active region. Other regions504, 510 and 514 of the sensor 500 have much wider band-gap than thequantum well 512 and their carrier concentrations can be considered tobe constant.

[0167] The n⁺ layers 520 act as source and drain electrodes betweenwhich there is a conducting path via the p ⁻ layer 514 and the quantumwell 512. It is the resistance of this conducting path that is magneticfield dependent and enables magnetic field to be measured.

[0168] In the sensor 500 the carrier concentration change over 50K isless than 1%; this is a very high degree of constancy and suitable fordemanding applications. It performs better in this regard compared toearlier embodiments because the quantum well carrier concentration isdetermined by modulation doping, which is a fixed parameter unlikethermal activation of electron-hole pairs.

[0169] The layer structures shown in FIG. 6, 15 and 16 may each be usedto make both Hall effect and magneto-resistance sensors. The differencebetween the two types of sensor is simply that the former has a fourterminal configuration as in FIG. 1 and the latter a two terminalconfiguration corresponding to absence (or non-use) of limbs 14 b and 14d.

1. A magnetic field sensor incorporating a semiconducting sensor element(10, 50) with an active region (14 e, 53 e) in which a signal responsiveto magnetic field is developed during operation, characterised in thatthe sensor element (10, 50): a) is in an at least partially intrinsicconduction regime when unbiased and at a normal operating temperature;b) includes a junction (36, 110) which is biasable to reduce intrinsicconduction in the active region (14 e, 53 e) and confine charge carrierspredominantly to one type only corresponding to an extrinsic saturatedregime, and c) includes means for detecting a signal developed in theactive region (14 e, 53 e) in response to applied magnetic field.
 2. Asensor according to claim 1 characterised in that the junction is anexcluding contact (36) for exclusion of minority carriers from theactive region (14 e).
 3. A sensor according to claim 2 characterised inthat the excluding contact (36) is a homojunction between indiumantimonide materials of different doping concentration.
 4. A sensoraccording to claim 2 characterised in that the excluding contact (36) isa heterojunction between indium antimonide (12) and a material (40, 41)having a wider band gap than indium antimonide.
 5. A sensor according toclaim 1 characterised in that it is a cruciform Hall effect sensor witha central active region (14 e) from which four limbs (14 a to 14 d)extend, at least one limb (eg 14 a) is connected to an excluding contact(eg 16 a) for depleting the active region's minority carrierconcentration when biased, a first pair of limbs (14 a, 14 c) isconnectable to a current supply and a second pair of limbs (14 b, 14 d)is connectable to Hall voltage measuring apparatus.
 6. A sensoraccording to claim 5 characterised in that each of the second pair oflimbs (53 b, 53 d) has a tapering portion adjacent the central activeregion (53 e).
 7. A sensor according to claim 5 characterised in thateach limb (eg 14 a) is connected to a respective excluding contact (eg16 a).
 8. A sensor according to claim 1 characterised in that thejunction is an extracting junction (110) for extraction of minoritycarriers from the active region (53 e).
 9. A sensor according to claim 8characterised in that the extracting junction (110) is a junctionbetween two sensor regions of materials having different majoritycarrier type and different band gap.
 10. A sensor according to claim 8characterised in that the extracting junction (110) is: a) sufficientlythick to prevent electron tunnelling through it; and b) sufficientlythin to avoid relaxing strain in materials associated with it.
 11. Asensor according to claim 9 characterised in that the extractingjunction (110) is a heterojunction of n-type indium antimonide (53) andIn_(1-x)Al_(x)Sb (54) where x is in the range 0.10 to 0.5.
 12. A sensoraccording to claim 10 characterised in that x is in the range 0.15 to0.2.
 13. A sensor according to claim 11 characterised in that xsubstantially 0.15.
 14. A sensor according to claim 8 characterised init is cruciform with a central active region (53 e) from which fourlimbs (53 a to 53 d) extend, the limbs have four successively disposedlayers (64, 66, 53, 54) of which two adjacent layers are of one majoritycarrier type and two other adjacent layers are of the other majoritycarrier type, the extracting junction (110) is a heterojunction betweenan active region layer (53) and another layer (66) of different band gapand majority carrier type, a first pair of limbs (14 a, 14 c) isconnectable to a current supply, a second pair of limbs (14 b, 14 d) isconnectable to Hall voltage measuring apparatus, and the extractingjunction (110) is reverse-biasable by means of a sensor substrateconnection.
 15. A sensor according to claim 14 characterised in that thefour successively disposed layers (64, 66, 53, 54) are an n⁺npp⁺structure.
 16. A sensor according to claim 14 characterised in that eachlimb (53 b, 53 d) of the second pair of limbs has a tapering portionadjacent the central region (53 e).
 17. A sensor according to claim 14characterised in that each limb (53 b, 53 d) of the second pair of limbshas a contact region adjoining onto the central active region (53 e)which less than 10% of the limb width of each limb of the first pair oflimbs (53 a, 53 c).
 18. A sensor according to claim 8 characterised inthat it is arranged for extraction current flow in a directionsubstantially orthogonal to charge carrier deflection by a magneticfield in normal operation.
 19. A sensor according to claim 1characterised in that the junction (514/520) is extracting and theactive region is p-type.
 20. A sensor according to claim 1 characterisedin that the active region is a quantum well structure (510/512/514). 21.A sensor according to claim 20 characterised in that it includes aδ-doping layer arranged to be a dominant source of charge carriers forthe quantum well structure (510/512/514).
 22. A sensor according toclaim 20 or 21 characterised in that it is a n⁺-p ³¹ -quantum well-p ³¹-p ⁺ diode structure.
 23. A sensor according to claim 1 characterised inthat it includes a δ-doping layer arranged to be a dominant source ofcharge carriers for the active region.
 24. A sensor according to claim23 characterised in that it is an n⁺-p-p ⁺-p⁺ diode structure.
 25. Amethod of detecting a magnetic field characterised in that it includesthe steps of: a) providing a magnetic field sensor incorporating asemiconducting sensor-element (10, 50) with an active region (14 e, 53e) in which a signal responsive to magnetic field is developed duringoperation, the sensor element (10, 50) being in an at least partiallyintrinsic conduction regime at a normal operating temperature whenunbiased, and including a junction (36, 110) which is biasable to reducean intrinsic contribution to conduction in the active region (14 e, 53e) and confine charge carriers predominantly to one type onlycorresponding to an extrinsic saturated regime; b) biasing the sensoractive region and the junction (36, 110) to provide for charge carrierflow in the active region and sensor operation to correspond to anextrinsic saturated regime and applying a magnetic field to the activeregion (14 e, 53 e); and c) detecting a signal developed by the activeregion (14 e, 53 e) at least partially in response to the magneticfield.
 26. A method of detecting a magnetic field according to claim 25characterised in that the step of biasing the sensor active region iscarried out at constant voltage and the step of detecting a signalinvolves detecting a voltage signal.
 27. A method of detecting amagnetic field according to claim 25 characterised in that the sensor isa Hall effect sensor, the step of biasing the sensor active regioninvolves applying a constant current thereto and the step of detecting asignal involves detecting a current signal.